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Celestial Equator

StarDate:

January 31, 2015

Dashing Orion, the hunter, climbs high across the south this evening. Look for his three-star belt, which forms a short diagonal line.

The star at the top of the belt lies almost directly atop the celestial equator — the projection of Earth’s equator into the sky. Just as the equator separates Earth into northern and southern hemispheres, the celestial equator separates the great sphere of the sky into two halves.

From Earth’s equator, you can see both halves. All that’s missing are the stars that are quite near the celestial poles. Polaris, which marks the north pole, would be right on the horizon, so it wouldn’t be visible.

As you move away from the equator, though, the view of the heavens constricts. From 30 degrees north, for example — the latitude of Austin — you can’t see anything that’s within 30 degrees of the south celestial pole, because those stars are always below the horizon.

And if you stand atop the north pole, your view is limited to only the northern celestial hemisphere. That means that half of the universe is blocked from view.

That has a practical effect for astronomy. To see the entire universe, astronomers need telescopes both north and south of the equator. In the last few decades, in fact, astronomers in the U.S. and Europe have worked with local scientists to build several observatories in South America and Africa — facilities that help us all appreciate the wonders of the entire universe.

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